1. Field of the Invention
The present invention relates to a radiation image processing apparatus and a processing method for extraction or removal of a specific object in a radiation image using a plurality of pieces of radiation image information acquired by applying radiation of different energies to a subject.
2. Description of the Related Art
In the medical field, for example, extensive use is made of radiation image processing apparatus which expose a subject (patient) to radiation emitted from a radiation source, guide the radiation that has passed through the subject to a radiation converting panel for converting radiation into radiation image information, and then perform predetermined image processing on the radiation image information. Processed radiation image information is displayed on a display unit and can be used for diagnosis etc.
One example of the radiation converting panel is a solid-state detector that converts radiation into charge information and stores the charge information so that it can be read out as an electric signal. Another example of the radiation converting panel is a stimulable phosphor panel. The stimulable phosphor panel stores radiation energy in a phosphor and emits stimulated light of an intensity corresponding to the stored energy when irradiated with stimulating light such as a laser beam.
One practical application of the radiation image apparatus is the extraction of a region of interest in the subject, e.g., soft tissue such as the heart and lungs located under ribs, from the radiation image information. The extraction of the region of interest is achieved based on a difference in absorption characteristics between bone such as the ribs and soft tissue such as the heart. Radiation of different energies is applied to the subject using two different image capturing conditions to acquire two pieces of radiation image information. The extraction of the bone or soft tissue of interest is achieved by computing the difference between the two pieces of radiation image information with predetermined weighting coefficients.
Since different internal structures of the subject have different radiation absorption characteristics, it is necessary to process the image in accordance with these characteristics to acquire a proper image of the region of interest.
Japanese Laid-Open Patent Publications Nos. 2003-037778 and 2003-244542 disclose image processing method in which the extraction of bone or soft tissue is achieved using a predetermined relation between radiation source tube voltages, which determine the radiation dose to the subject, and weighting coefficients used for computing a difference between the pieces of radiation image information. The image processing method disclosed in Japanese Laid-Open Patent Publication No. 2002-330954, acquires a first piece of radiation image information by applying radiation to a subject in accordance with a predetermined image capturing condition. Then, the image capturing condition is modified based on analysis of the first piece of radiation image information, and used for acquisition of a second piece of radiation image information. Finally, an image of a region of interest is produced from the first and second pieces of radiation image information.
The methods disclosed in Japanese Laid-Open Patent Publications Nos. 2003-037778 and 2003-244542, however, require highly experienced operators for the determination of a suitable tube voltage and weighting coefficient for a region of interest. Further, the above-mentioned Publications are only concerned with the tissue of a subject, and no consideration is given to methods for extraction or removal of a foreign body within a subject. The method disclosed in Japanese Laid-Open Patent Publication No. 2002-330954, which determines the image capturing condition for the second piece of radiation image information based on the first piece of radiation image information, may fail to obtain proper image information when the subject moves before the acquisition of the second piece of radiation image information.